Medical ventilators, of the type providing a continuous gas flow, are widely used. In such ventilators, breathing gas flows continuously or nearly continuously, through a patient circuit having an inspiratory leg and an expiratory leg, coupled by a fitting or patient adaptor, which is in communication with the patient's breathing passages. This adaptor is commonly referred to as a "tee" or a "wye," depending upon its particular configuration. Thus, the patient, breathing through the adaptor, receives gas from the inspiratory leg during inhalation, and exhales through the adaptor into the expiratory leg during exhalation.
The expiratory leg commonly has an exhalation valve which is open only during exhalation. Thus, during inhalation, gas flows only to the patient. At the end of the inspiratory phase of the patient's respiration, the exhalation valve opens, allowing gas to flow from the patient, through the exhalation valve, and out the exhaust port of the ventilator. During exhalation, gas continues to flow from the inspiratory leg to the expiratory leg through the patient adaptor. This latter flow continues in this manner until the start of the next inhalation phase.
An undesirable characteristic of such a system is that the pressure at the patient adaptor is always slightly higher than the ambient pressure (at the exhaust port), even after exhalation is complete. Thus, the patient will experience a residual positive end expiratory pressure, which is referred to as "inadvertent PEEP." Although a number of factors contribute to inadvertent or residual PEEP, the dominant factor is usually the pressure drop that the gas undergoes as it passes through the expiratory leg due to the pneumatic resistance of the expiratory leg components, including the exhalation valve.
While there are therapeutic regimens which require some degree of PEEP, in many cases PEEP is not desired. Moreover, in those cases where PEEP is desired, the requirement is to control the degree of PEEP carefully. Thus, inadvertent PEEP can not only impair the ability to operate without PEEP, but it can also make difficult the achievement of a precisely-controlled PEEP.
Although conventional pressure-compensating means can be used to minimize inadvertent PEEP, such means are effective only for relatively narrow ranges of ventilator operational parameters, e.g., flow rate and expiratory leg pneumatic resistance. This is due to the variation in inadvertent PEEP which results from variations in these machine characteristics. Thus, for a given patient, inadvertent PEEP will vary proportionately with ventilator flow rate or expiratory leg pneumatic resistance.
It has long been known that the exhalation phase of respiration can be aided or assisted by the use of a venturi jet pump in the patient circuit. Typically, the jet pump is used to create a negative pressure at the patient connection so that gas is extracted from the patient's lungs. See, for example, U.S. Pat. Nos. 3,465,752--Brychta et al.; 2,408,136--Fox; 2,376,348--Fox; 3,073,298--Stanton; and 3,485,243--Bird et al. However, none of the devices disclosed in these patents addresses the problem of minimizing inadvertent PEEP over a wide range of flow rates in a continuous flow ventilator system.
This problem has been addressed using a system such as that disclosed in U.S. Pat. No. 3,842,828--Bird. This system employs a venturi jet pump in the expiratory leg of the patient circuit, between the patient adaptor and the exhalation valve. The gas provided to the nozzle of the jet pump is supplied separately from the gas in the patient circuit, and thus has no inherent relationship to the gas flow rate in the patient circuit. Accordingly, adjustment of the negative pressure provided by the jet pump to compensate for variations in inadvertent PEEP due to changes in patient circuit flow rate is effected by varying the flow rate of the gas supplied to the jet pump nozzle by means of a manually-activated flow rate control valve.
From the foregoing, it can be appreciated that the ventilator art would benefit from a mechanism which minimizes inadvertent PEEP over a wide range of patient circuit flow rates, and which does so without the need for a separate pneumatic circuit having its own controls which must be monitored and manually adjusted. Moreover, it would also be beneficial to provide a mechanism which achieves such operational goals, while also being simple to construct and use, and which is also easily adaptable to widely varying operational characteristics.